1540s, "group or set of three," from Late Latin trias (genitive triadis), from Greek trias (genitive triados) "a triad, the number three," from treis "three" (see three). Musical sense of "chord of three notes" is from 1801. Related: Triadic.
trestle
tret
Trevor
trey
tri-
triad
triage
trial
triangle
triangular
triangularity